Calculating the Loading

We can calculate the loading for a given 10G link quite simply. If the average packet rate is 1.5Mpps, and the average packet size is 350 bytes, the ethernet frame size will be 350+18 = 368, and with overheads the average transmission size will be 368 + 20 = 388 bytes. So the loading will be:

1.5Mpps * 388 bytes * 8 bits / 10Gbps = 46.56%

On The Wire

At the physical level, ethernet traffic is broken into blocks and encoded before transmission, with the reverse happening on receipt. This process is invisible to both the user and the operating system, being handled in the network adapters at each end of the link. Standard 10G ethernet is sent serially over fibre, using on-off keying and 66b/64b encoding. So the actual bit rate transmitted is:

10G x (66 / 64) = 10.3125Gbps

40G ethernet is sent as 4 lanes of 10G data. Over multi-mode fibre, each of these lanes runs over a separate fibre pair, typically bundled into a MPO cable. Over single-mode fibre, the lanes are transmitted with different frequency lasers multiplexed onto the same fibre pair – known as CWDM (coarse wave-division multiplexing).

Latency Calculations

As data rates increase, the latency introduced by even fairly short lengths of fibre can become quite significant. They can be calculated as follows:

the speed of light in the fibre = the speed of light in a vacuum / the refractive index of the fibre

The speed of light in a vacuum is 3 x 10^8 m/s, and the refractive index of fibre is typically 1.5. So:

the speed of light in fibre = 3 x 10^8 / 1.5 = 2 x 10^8 m/s

And the delay per metre is:

delay = 1 / 2 x 10^8 m/s = 5ns per metre.

So two 10m lengths of fibre will introduce a 100ns latency, in addition to the delay through the NICs and switch as a packet moves from one server to another.

Call Andrew Thomson today to find out more:
+44 (0) 800 852 7671


When taking packet captures for analysis, it is helpful to have an estimate of how much data will be seen in a particular period, and how rapidly it will need to be written to disk. The table below shows the maximum amount of data that can be transmitted over different link types in varying periods of time, and the maximum per-second data rate that could be seen.

Call Andrew Thomson today to find out more:
+44 (0) 800 852 7671